Exhaust gas recirculation (EGR) systems divert a portion of the exhaust gases back to the intake to cool combustion temperatures and reduce throttling losses, thus improving vehicle emissions and fuel economy. In turbocharged engines, an EGR system may include a low-pressure EGR (LP-EGR) circuit, a high-pressure EGR (HP-EGR) circuit, or both. The LP-EGR circuit diverts exhaust gases after the gases pass through the turbine of the turbocharger and injects the gases before the compressor, while the HP-EGR circuit diverts exhaust gases before the turbine and injects the gases after the intake throttle. Traditionally, the amount of LP-EGR and/or HP-EGR routed through the EGR system is measured and adjusted based on engine speed and load during engine operation to maintain desirable combustion stability of the engine while providing emissions and fuel economy benefits.
However, the inventors herein have recognized an issue with the above approach. The LP-EGR circuit has a long transport delay, as the exhaust gases must travel through the turbo compressor, high pressure air induction plumbing, charge air cooler, and intake manifold before reaching the combustion chamber. As a result, it may be difficult to provide the desired amount of EGR to the cylinders, particularly during transient conditions. This is due to the fact that, by the time the EGR reaches the cylinder, the engine speed/load state may have changed and another EGR rate may be desired.
Thus, in one example, the above issues may be at least partially addressed by a method for controlling EGR flow in a turbocharged engine. The method comprises operating a low-pressure exhaust gas recirculation (LP-EGR) system at a fixed EGR percentage rate of fresh airflow from mid-load down to a minimum engine load, even as load, and thus airflow or air charge, changes.
In this way, as opposed to constantly changing the LP-EGR rate relative to the air, the LP-EGR circuit may have a fixed EGR percentage rate of fresh airflow across an area of the speed/load map, including a minimum engine load corresponding to closed throttle (e.g., due to drive pedal tip-outs). In one embodiment, a constant percent of LP-EGR may be provided in the areas most likely to experience problematic transient control of LP-EGR, such as the minimum load encountered during a driver pedal tip-out event.
The present description may provide several advantages. Because transient control issues may reduce the areas of the speed-load map under which EGR may be utilized effectively, improving transient control may extend the use of EGR under more operating conditions, reducing throttling losses and improving fuel economy. Further, EGR lowers peak cylinder temperatures, reducing the production of NOx. Thus, extending the use of EGR into more areas of the speed-load map may reduce engine-out emissions. Also, operating the LP-EGR circuit with a fixed percentage rate of fresh airflow may improve component durability. For example, not turning LP-EGR on and off during tip-outs and tip-ins reduces the number of thermal cycles on the EGR cooler, improving its durability. Additionally, the number of closing events of the EGR valve may be reduced, improving valve durability. Next, EGR mixing may be improved, as running a consistent LP-EGR amount can allow for optimized sizing of the EGR and air flow ducts to create even dispersion of the EGR into the air. Finally, running fixed LP-EGR may reduce the dynamic range requirements for the EGR valve and sensors and simplify the control strategy for the EGR throttle, reducing system costs and complexity.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.